Profinite groups, profinite completions and a conjecture of Moore

Abstract

Let R be any ring (with 1), \Gamma a group and R\Gamma the corresponding group ring. Let H be a subgroup of \Gamma of finite index. Let M be an R\Gamma -module, whose restriction to RH is projective. Moore's conjecture: Assume for every nontrivial element x in \Gamma, at least one of the following two conditions holds: M1) the subgroup generated by x intersects H non-trivially (in particular this holds if \Gamma is torsion free). M2) ord(x) is finite and invertible in R. Then M is projective as an R\Gamma-module. More generally, the conjecture has been formulated for crossed products R*\Gamma and even for strongly graded rings R(\Gamma). We prove the conjecture for new families of groups, in particular for groups whose profinite completion is torsion free. The conjecture can be formulated for profinite modules M over complete groups rings [[R\Gamma ]] where R is a profinite ring and \Gamma a profinite group. We prove the conjecture for arbitrary profinite groups. This implies Serre's theorem on cohomological dimension of profinite groups.